This is my first "mix" ever. So, to make it as good as I can, I spend
forever listening to the mix, making decisions about what needs to be changed, even taking
notes. On multiple sets of speakers!

But ... when I wake up and listen again
- someone has gone into Cubase, changed all the balances, EQs, and send levels, done a new
mixdown and replaced my .wav file with a new one! It all sounds so different than it did
the day before, I can't think of any other explanation!!

(In all seriousness, does this mean I need to stop sooner then night before, before my
brain starts playing tricks after being subjected to intense listening for so long? If so
... how do I recognize when that magic tipping point in time is (I'd rather be sleeping if
I'm not actually making any progress!)?

Yeah that's very common. You need to resist the temptation to fiddle with things late in
the day, that sounded good earlier on. Build a solid foundation with your fundamental
balance of drums, bass, vocal and any other primary instrument (like guitars in rock for
instance) and stick with it. Ride stuff, automate, play with fx etc but resist the urge to
go round in circles with your original balance. If you listen in the morning and its
wrong, start again. When you feel that temptation it might be a good time to stop and get
some perspective.

Reference material. I was fiddling with a work in progress and thinking that it was better
than I remembered it being. Needs some work but sounding ok. Wanted to patch in a vocoder
in Logic and couldn' remember, so saved it, opened the Lilly Allen song and checked the
routing. Back to mine and got the vocoder plugged in. Then noticed that everything else
was rather dull sounding. Ah...

Pick something that you know well and after a
break, have a short blast of that and see if helps reset your ears.

The other
thing that I've done recently is invest in a decent pair of headphones. I had some mixes
that I thought sounded ok on my main monitors, lesser headphones and inbuilt iMac
speakers, but one listen through the new cans and I zeroed in on why some people had made
comments about the drum sound - snare was a little brittle and things generally could be
smoother. Once I'd heard it, I could understand my monitors better and can improve on it
next time.

You want to make sure you're monitoring at sensible volumes and take regular breaks. And
as others have suggested, spend some time getting used to your monitoring system and build
a collection of reference material. Have a read of this article.

Your ears are adaptable sensors, not absolute. You need a few references (certain mixes
heard from the same system) to keep yourself grounded. Your ears will change with the
music. That's what they do.

My rule is never to sign off a mix after supper. I can't tell you how often that's bailed
me out. Although it is complicated a little when I'm so absorbed in what I'm doing that I
forget to have supper...

I think this issue might also be the reason why Bruce Swedien always keeps his first
rough mix to hand while doing the final mixdown -- it keeps you in touch with your
instinctive balances from before you got the chance to overthink them.

I'm also a great believer in saving regular backups of your mix as you go.

Even if your 'final' mix ends up being the one you love, it can be a very educational
experience rendering some of the older ones down to stereo and then importing them into
your final multi-track project file - just mute these other imported tracks and so you
just hear your final multi-track extragaganza, then solo the imports one at a time to
switch instantly between a handful of mixes chronologically to hear the differences
between them.

I guarantee you'll find the results fascinating, but
potentially time-consuming when you realise that some of your earlier mixes had a few
better aspects than the final mix, even though that may be better overall

The long you spend on one mix, the more the benchmark slips. You lose your reference
point, it's like the longer you drive on the interstate, the slower 75mph appears to you.
It's only when you hit the exit ramp at 65mph that you suddenly get your perspective back.
If I overdo it my mixes are always way too bright in the cold light of day.

Drove on the I-10 once. Satnav said 'In 427 miles, keep right'. You must need good
memories out in Texas! Still makes me smile.